Confined synthesis of three-dimensionally ordered mesoporous-imprinted zeolites with tunable morphology and Si/Al ratio

Three dimensionally ordered mesoporous-imprinted (3DOm-i) zeolites with well-controlled mesoporosity and Si/Al ratio were synthesized within the confined space of three dimensionally ordered mesoporous (3DOm) carbon by conventional hydrothermal synthesis method. It was found that the incipient wetness technique used to prepare the 3DOm carbon can significantly affect the mesoporous structure of the resulting carbon template. 3DOm carbon with a nearly complete replication from silica colloidal crystals can be used to control the mesoporosity of 3DOm-i zeolites. Confined synthesis of hierarchical zeolites under conventional hydrothermal synthesis conditions offers a large variety of synthesis recipes for controlling the framework, morphology and composition of 3DOm-i zeolites. It shows that a dilute initial synthesis solution favors the formation of single crystalline 3DOm-i silicalite-1, while a higher starting silica concentration results in polycrystalline 3DOm-i silicalite-1. The polycrystalline feature might benefit the disassembly of the resulting 3DOm-i zeolites for the preparation of highly monodisperse zeolite nanocrystals. 3DOm-i ZSM-5 with tunable acid site concentrations was also achieved by varying Si/Al ratios in initial synthesis solutions. The catalytic performance of 3DOm-i ZSM-5 was tested with the self-etherification of benzyl alcohol, a reaction severely limited by the mass transport of reactants and products in the micropores of ZSM-5 catalysts. It was revealed that for this reaction 3DOm-i structure can significantly improve the reaction rate by alleviating diffusion limitation, and the Si/Al ratio of 3DOm-i ZSM-5 also plays an important role in the effective utilization of the zeolite catalysts. (c) 2013 Elsevier Inc. All rights reserved.